Recently, a plastic crystal having high conductivity has drawn public attention as a new type of electrolyte material because such a plastic crystal offers the potential of solidifying the electrolyte used in electrochemical devices, including primary and secondary batteries, such as lithium secondary batteries; dye-sensitized solar cells; and fuel cells. Known materials are limited in terms of having a practical level of high conductivity and/or a usable temperature range.
A plastic crystal is a mesophase that is formed by a first-order solid to solid phase transition at a temperature lower than the melting point of the molecule or ionic compound. In this mesophase, the molecules or ions that form the plastic crystalline phase are characterized by their long-range order regarding position, and rotational or disorder for orientation. This type of disorder results in the formation of defects. However, such defects not only create liquid-like characteristics that allow doped ions or matrix-forming ions to quickly migrate, but also render these materials with plastic-like mechanical characteristics. A lithium ion conductive plastic crystal that is usable as a solid electrolyte for lithium batteries first became known through a report showing that a lithium-ammonium double salt exhibits high conductivity. It was reported that the conductivity was dramatically increased by two or more orders of magnitude by doping the plastic crystalline phase of a pyrrolidinium-TFSI salt with a small amount of lithium bis(trifluoromethanesulfonyl)imide (TFSI−). As a result of this, plastic crystalline electrolytes have recently been drawing attention (Non-Patent Documents 1 to 3).
Plastic crystals used as a solid matrix for dissolving lithium salt can be divided into two categories: molecules and ions. A plastic crystalline electrolyte having a high conductivity of 10−4 to 10−3 Scm−1 at room temperature has been obtained by doping a plastic crystalline phase with succinonitrile [NC(CH2)2CN]. Initial reports indicated the possibility of using that plastic crystalline electrolyte as a solid electrolyte material for lithium batteries; however, such a material contains a large amount of combustible molecular compounds and therefore has drawbacks (e.g., volatility and combustibility) that are similar to those of conventional liquid electrolytes. In contrast, an organic ion plastic crystal (OIPC) has a relatively low melting point (no higher than 100° C.), and exhibits excellent ionic liquid characteristics, such as noncombustibility. These advantages were fully understood, but the heretofore reported lithium ion conductive OIPC has a low ion conductivity at room temperature and this hinders the lithium ion conductive OIPC from being practically used in lithium batteries.
The present inventors reported that N,N-diethyl-N-methyl-N-(n-propyl)ammonium trifluoromethyl trifluoroborate was a plastic crystal; however, its electroconductivity at room temperature was as low as about 10−6 Scm−1 (Non-Patent Document 4).
Non-Patent Document 1: D. R. MacFarlane, J. Huang, M. Forsyth, Nature 402 (1999) 792.
Non-Patent Document 2: D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, M. Forsyth, J. Phys. Chem. B 103 (1999) 4164.
Non-Patent Document 3: M. Forsyth, J. Huang, D. R. MacFarlane, J. Mater. Chem. 10 (2000) 2259.
Non-Patent Document 4: Z.-B. Zhou, H. Matsumoto, K. Tatsumi, Chem. Eur. J. 11 (2005) 752.